Semiconductor chip or die automated assembly equipments typically rely on the use of vacuum operated placement heads often referred to as vacuum grippers or pick-and-place tools. In their simplest embodiment, these placement heads typically consist of an open ended cylinder having a drilled nozzle surface which seals to the die to accomplish physical attachment. Semiconductor chips or die which are ultra thin, fragile, or too small cannot be economically handled by conventional vacuum grippers. As a result, alternative approaches such as self-assembly or dry transfer printing technologies are being investigated.
Transfer printing enables the massively parallel assembly of high performance semiconductor devices onto virtually any substrate material, including glass, plastics, metals or other semiconductors (see, e.g., U.S. patent application Ser. No. 11/145,574 entitled “METHODS AND DEVICES FOR FABRICATING AND ASSEMBLING PRINTABLE SEMICONDUCTOR ELEMENTS,” filed Jun. 2, 2005). This semiconductor transfer printing technology relies on the use of a microstructured elastomeric stamp to selectively pick-up devices from a source wafer, and then prints these devices onto a target substrate. The transfer process is massively parallel as the stamps are designed to transfer hundreds to thousands of discrete structures in a single pick-up and print operation.
While pick-and-place tools rely on suction forces, dry transfer printing tools rely on surface adhesion forces to control the pickup and release of the semiconductor devices. To enable dry transfer printing, methods to control the adhesion forces between the semiconductor elements and the elastomeric stamp are required. One such method is described in U.S. patent application Ser. No. 11/423,192 filed Jun. 9, 2006, entitled “PATTERN TRANSFER PRINTING BY KINETIC CONTROL OF ADHESION TO AN ELASTOMERIC STAMP.” In that method, the elastomeric stamp adhesion forces are controlled by adjusting the delamination rate of the elastomeric transfer stamp. This control of separation or delamination rate provides a means of increasing the stamp adhesion forces that may be used to pickup semiconductor elements from a source wafer. There may be problems, however, associated with transferring the semiconductor elements from the stamp to a receiving substrate with this technique. For example, stamps optimized for dry transfer printing semiconductor elements with high placement accuracy typically use a stiff backing layer. During the printing or transfer step, the delamination rate of those stamps can be unstable and/or difficult to control when the stiff backing layer(s) are subject to bending forces. Also, printing yields on surfaces that are not ultra smooth, and/or on low tack surfaces, can be very low.
Accordingly, there is a need for improved methods for transfer printing semiconductor elements.